Method and apparatus for a porous electrospray emitter

ABSTRACT

An ionic liquid ion source can include a microfabricated body including a base and a tip. The body can be formed of a porous material compatible with at least one of an ionic liquid or room-temperature molten salt. The body can have a pore size gradient that decreases from the base of the body to the tip of the body, such that the at least one of an ionic liquid or room-temperature molten salt is capable of being transported through capillarity from the base to the tip.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.16/257,786, filed Jan. 25, 2019, which is a continuation of U.S.application Ser. No. 15/805,050, filed Nov. 6, 2017, which is acontinuation of U.S. application Ser. No. 15/272,574, filed Sep. 22,2016, which is a continuation of U.S. application Ser. No. 14/336,814,filed Jul. 21, 2014, which is a continuation of U.S. application Ser.No. 14/070,351, filed Nov. 1, 2013, which is acontinuation-in-part-of—U.S. application Ser. No. 13/839,064, filed Mar.15, 2013, which is a continuation-in-part of U.S. patent applicationSer. No. 13/681,155, filed on Nov. 19, 2012, which is a continuation ofU.S. patent application Ser. No. 12/990,923, filed on May 3, 2011, whichis a national stage filing under 35 U.S.C. § 371 of InternationalApplication No. PCT/US2009/042990, filed on May 6, 2009, which claimsthe benefit of priority under 35 U.S.C. § 119(e) to U.S. ProvisionalPatent Application No. 61/050,847 filed on May 6, 2008, the contents ofeach of which are incorporated herein by reference in their entirety.U.S. application Ser. No. 13/839,064, filed Mar. 15, 2013, also claimsthe benefit of priority under 35 U.S.C. § 119(e) to U.S. PatentApplication No. 61/695,034, filed Aug. 30, 2012, the contents of whichare incorporated herein by reference in their entirety.

GOVERNMENT SUPPORT

This invention was made with Government support under Grant No.FA2386-11-1-4074 awarded by the Asian Office of Aerospace Research andDevelopment (AOARD). The Government has certain rights in the invention.

FIELD OF THE INVENTION

The technology generally relates to devices and methods of generatingions. More specifically, the invention relates to methods and devicesfor an electrospray emitter.

BACKGROUND OF THE INVENTION

Existing colloid thrusters utilize pressure fed capillary emittergeometry to transport liquid to the base of Taylor Cones. FIG. 1 shows aschematic of Taylor Cone formation from a pressure fed capillaryemitter. A voltage can be applied to a capillary emitter 10, relative toan electrode 20. The balance between surface tension and electricpressure forms a Taylor Cone 30 and generates emission of ions 40.Droplets can be emitted, due to instability, at apex of cone 50.Droplets can carry most of the ejected mass (i.e., since droplets arerelatively heavy) while delivering little impulse (i.e., as dropletsmove relatively slowly). This can translate into inefficient operation.In ion beam etching, droplets can also contaminate the substrate.

Pressure fed capillary emitters, however, can require pressurizationsystems (e.g., onboard the spacecraft using the emitters), that addsmass/weight and complexity to the system. The difficulties infabricating small, uniform capillaries can pose problems in theminiaturization of needle arrays. One way to avoid the issues ofpressure fed capillary emitters is to use externally wetted emittergeometries where liquid is drawn from a reservoir by capillary forces.Such passively fed systems can supply liquid at the rate established bythe electrospray emission process. The use of externally fed emitters invacuum, however, is possible with ionic liquids.

Ionic liquids (ILs) are molten salts at room temperature and exhibitextremely low vapor pressures. ILs are formed by positive and negativeions which can be directly extracted and accelerated to produce thrustwhen used in bipolar operation. ILs have been shown to emit a purelyionic current when exposed to a strong applied potential. ILs generate asubstantially pure ionic emission and have a relatively low startingvoltage (e.g., less than approximately 2 kV required to generate ionsfrom the Taylor Cone). ILs allow for a scalable specific impulse of theelectrospray emitter(s) from approximately 500 seconds to 5000+ seconds.Some ILs can display super-cooling tendencies in which they remain asliquids well below their nominal freezing points. Just as theirinorganic cousins (simple salts like NaCl, KBr, etc.) at their meltingpoints (typically >850° C.), ILs exhibit appreciable electricalconductivity at room temperature, making them suitable for electrostaticdeformation and subsequent Taylor Cone formation. ILs are thermallystable over a wide range of temperatures (they do not boil, butdecompose at temperatures ˜250-500° C.) and are apparently non-toxicbeing able to be used with applications with green standards, such as inthe synthesis and catalysis of chemical reactions. ILs can be used inelectrochemical systems, such as in high energy densitysuper-capacitors. ILs' electrochemical window (i.e., the maximumpotential difference sustainable by the liquid before electrochemicalreactions are triggered) is higher than in conventional aqueoussolutions. ILs have low vapor pressures at, or moderately above, theirmelting points. This allows for use in high vacuum equipment in openarchitectures such as externally wetted needles/emitters.

Ion sources using ILs can produce positive or negative ion beams with:(1) narrow energy distributions, (2) high brightness, (3) small sourcesize, and (4) wide selection of liquids with diverse molecularcompositions. IL ionic sources can be used as a simple and compactsource of nearly-monoenergetic negative ions, which can reduce thecharge build-up that limits the ability to focus non-neutralizedpositive ion beams onto dielectrics (insulators or some biologicalsamples) or conductive, but electrically floating targets, and act as achemically reactive etch agent for materials micro- and nanoprocessingapplications.

SUMMARY OF THE INVENTION

Porous metal Electrospray emitters have been shown to emit more currentthan a comparably sized solid externally wetted emitter (e.g., needle),due to the increased capillary flow capacity (e.g., greater flow throughvolume) provided by the volumetric porosity of the emitter substrate.Porous metal emitters also have the benefit of being a passive,self-regulating capillary supply that reduces complexity over pressurefed capillary systems coupled with the benefit of increased flow throughvolume that permits the porous metal emitters to emit greater currentand provide greater thrust. A typical thrust of a single porous metalemitter operating in the ionic mode can be about 0.05-0.1 μN/μA.Passively fed porous metal Electrospray emitters can emit purely in theionic regime, which allows for high specific impulse (high ISP)operation and high polydispersive efficiency. Multiple emitters can begrouped to produce a desired amount of current, for example, in spaceapplications. Micro-fabrication techniques can be used to manufacture asingle emitter or an array of emitters. Porous metal electrosprayemitters can be manufactured using, for example, photolithography andelectrochemical etching.

In one aspect, an ionic liquid ion source includes a microfabricatedbody including a base and a tip and formed of a porous metal compatible(e.g., does not react or result in electrochemical decaying orcorrosion) with at least one of an ionic liquid, or room-temperaturemolten salt. The microfabricated body can have a pore size gradient thatdecreases from the base of the body to the tip of the body, such thatthe ionic liquid is capable of being transported through capillarityfrom the base to the tip.

In another aspect, an ionic liquid ion source includes a plurality ofemitters microfabricated from a porous metal compatible with at leastone of an ionic liquid, or room-temperature molten salt. Each emittercan have a pore size gradient that decreases from the base of theemitter to the tip of the emitter, such that the ionic liquid is capableof being transported through capillarity from the base to the tip ofeach emitter.

In yet another aspect, a system for producing ions includes a source ofat least one of ionic liquid or room-temperature molten salt and anarray of emitters microfabricated from a porous metal compatible withthe at least one of ionic liquid or room-temperature molten salt, whereeach emitter can have a pore size gradient that decreases from the baseof the emitter to the tip of the emitter such that the ionic liquid iscapable of being transported through capillarity from the base to thetip of each emitter. The system can also include an electrode positioneddownstream relative to the array of emitters and a power source forproviding a voltage to the array of emitters with respect to theelectrode.

In another aspect, a method for manufacturing an array of electrosprayemitters can include applying polyimide to a first side of a samplecomprising a porous metal compatible with an ionic liquid, applyingphotoresist to the first side of the sample and applying a transparencymask to the first side of the sample and exposing the sample to UV lightto form an emitter geometry pattern. The method can also includeremoving the photoresist from the sample, curing the sample to hardenthe polyimide, electrochemically etching the sample to form an emittergeometry and removing the polyimide resulting in an array ofelectrospray emitters. The method can include the step of treatingand/or processing a tip of each emitter to vary a pore size between eachtip and each base of each emitter in the array.

In yet another aspect, a method for manufacturing an ion emitter caninclude forming a body from a porous metal compatible with at least oneof an ionic liquid or room temperature molten salt, the body having apore size gradient that decreases from a first end of the body to asecond end of the body. The method can also include microfabricating thebody to form a base relative to the first end of the body and a tiprelative to the second end of the body, wherein the ionic liquid iscapable of being transported through capillarity from the base to thetip.

In another aspect, a method for manufacturing an ion source can includeforming an emitter geometry pattern on a unitary substrate comprising aporous metal compatible with at least one of an ionic liquid, orroom-temperature molten salt. The method can also includeelectrochemically etching the unitary substrate to form a plurality ofemitters, where each emitter comprises a base at the first end of thesubstrate and a tip at the second end of the substrate. A tip of eachemitter can be processed/treated to form a pore size gradient thatvaries from the base to the tip of each emitter.

In another aspect, an ionic liquid ion source includes a body includinga base and a tip and formed of a porous material compatible with atleast one of an ionic liquid or room-temperature molten salt, the bodyhaving a pore size gradient that decreases from the base of the body tothe tip of the body, such that the at least one of an ionic liquid orroom-temperature molten salt is capable of being transported throughcapillarity from the base to the tip.

In another aspect, an ionic liquid ion source includes a plurality ofemitters formed of a porous material compatible with at least one of anionic liquid or room-temperature molten salt, each emitter of theplurality of emitters having a pore size gradient that decreases from abase of the emitter to a tip of the emitter, such that the at least oneof an ionic liquid or room-temperature molten salt is capable of beingtransported through capillarity from the base to the tip of eachemitter.

In another aspect, a system for producing ions includes a source of atleast one of an ionic liquid or room-temperature molten salt. The systemincludes an array of emitters formed of a porous material compatiblewith the at least one of an ionic liquid or room-temperature moltensalt, each emitter having a pore size gradient that decreases from abase of the emitter to a tip of the emitter such that the at least oneof an ionic liquid or room-temperature molten salt is capable of beingtransported through capillarity from the base to the tip of eachemitter. The system includes an electrode positioned downstream relativeto the array of emitters. The system includes a power source forproviding a voltage to the array of emitters with respect to theelectrode.

In another aspect, an ionic liquid ion source includes an emitter bodyformed of a porous material, the emitter body externally wetted by atleast one of an ionic liquid or room-temperature molten salt. The ionicliquid ion source includes an electrode electrically connected to theemitter body via the at least one of an ionic liquid or room-temperaturemolten salt.

In another aspect, a system for producing ions includes a source of atleast one of an ionic liquid or room-temperature molten salt. The systemincludes a first electrode in contact with the ionic liquid orroom-temperature molten salt, the first electrode formed of a firstporous material compatible with the at least one of an ionic liquid orroom-temperature molten salt. The system includes a connecting memberadjacent to the first electrode, the connecting member formed of asecond porous material compatible with the at least one of an ionicliquid or room-temperature molten salt. The system includes an array ofemitters adjacent to the connecting member, the array of emitters formedof a third porous material compatible with the at least one of an ionicliquid or room-temperature molten salt. In the system, the firstelectrode, the connecting member, and the array of emitters have a poresize gradient that decreases from the first electrode to the array ofemitters such that the at least one of an ionic liquid orroom-temperature molten salt is capable of being transported throughcapillarity from the first electrode, through the at least a portion ofthe connecting member, and to the array of emitters.

In another aspect, a method for manufacturing an array of electrosprayemitters includes forming a substrate from a porous material compatiblewith at least one of an ionic liquid or room temperature molten salt,the substrate having a pore size gradient that decreases from a firstend of the body to a second end of the body. The method includesprocessing the substrate to form one or more emitters, wherein eachemitter of the one or more emitters comprises a base and a tip formed ofthe substrate, the base relative to the first end of the substrate andthe tip relative to the second end of the substrate, and wherein theionic liquid is capable of being transported through capillarity fromthe base to the tip.

In another aspect, a method for manufacturing an array of electrosprayemitters includes forming a substrate from a first porous materialcompatible with at least one of an ionic liquid or room temperaturemolten salt. The method includes selectively depositing a second porousmaterial compatible with the at least one of an ionic liquid or roomtemperature molten salt onto the substrate to form one or more emitters,wherein each emitter of the one or more emitters comprises a base and atip, wherein the ionic liquid is capable of being transported throughcapillarity from the base to the tip.

In other examples, any of the aspects above, or any apparatus or methoddescribed herein, can include one or more of the following features.

In some embodiments, ionic liquid is capable of being continuouslytransported through capillarity from the base of a microfabricated bodyto the tip of the microfabricated body. The body can be a cylindricalneedle. In some embodiments, the body is a flat ribbon-like needle. Thetip of the microfabricated body can be formed by electrochemicaletching. In some embodiments, a radius of curvature of the tip is about1-20 μm.

In some embodiments, the porous metal is at least one of tungsten,nickel, magnesium, molybdenum or titanium.

In some embodiments, an ion source includes a plurality of emitters andionic liquid is capable of being continuously transported throughcapillarity from the base to the tip of each emitter. The emitters canbe formed by electrochemical etching. In some embodiments, a spacingbetween the emitters is less than about 1 mm. In some embodiments, asystem for producing ions includes an array of emitters and ionic liquidis capable of being continuously transported through capillarity fromthe base to the tip of each emitter.

In some embodiments, the porous material includes a dielectric material.In some embodiments, the dielectric material includes at least one of aceramic material, a glass material or another oxide material. In someembodiments, the metal material includes at least one of tungsten,nickel, magnesium, molybdenum or titanium. In some embodiments, thethird porous material includes a metal material. In some embodiments,the metal material includes at least one of silver, stainless steel,tungsten, nickel, magnesium, molybdenum, titanium, any combinationthereof, or any thereof coated with a noble metal material. In someembodiments, the third porous material includes a dielectric material.In some embodiments, the dielectric material includes at least one of aceramic material, a glass material or another oxide material. In someembodiments, the first porous material includes at least one of silver,stainless steel, tungsten, nickel, magnesium, molybdenum, titanium, anycombination thereof, or any thereof coated with a noble metal material.In some embodiments, the system for producing ions includes anextraction grid positioned downstream relative to the array of emitters.In some embodiments, the system for producing ions includes anaccelerator grid positioned downstream relative to the extraction grid.

A method for manufacturing an array of emitters can include filling theporous metal with photoresist and exposing the porous metal with a UVlight to block pores of the porous metal to form the sample. In someembodiments, the method includes blocking the porous metal surface bythe uniform deposition of mono-layers of a compatible material (e.g.,compatible with the ionic liquids and the porous metal substrate anddoes not react or result in electrochemical decaying or corrosion) usingChemical Vapor Deposition (CVD) or Physical Vapor Deposition (PVD). Thestep of applying polyimide to the first side of the sample can includeprebaking the sample. The method can include developing the sample totransfer the emitter geometry pattern by removing positive photoresistand etching the polyimide.

In some embodiments, electrochemically etching the sample includes thestep of removing excess porous metal to form the emitter geometry. Thestep of electrochemically etching the sample can include etching thesample to form a conical emitter geometry. In some embodiments, theporous metal is at least one of tungsten, magnesium, molybdenum,titanium or nickel.

The method for manufacturing an ion emitter can include microfabricatinga body to form a base and a tip. In some embodiments, the ionic liquidis capable of being continuously transported through capillarity fromthe base to the tip. Microfabricating the body can include shaping thebody into a flat ribbon-like needle.

A surface of the tip of an emitter (e.g., one or more emitters in anarray) can be treated/processed by applying a layer of compatible metal(e.g., a porous layer of metal compatible with the ionic liquids and theporous metal substrate that does not react or result in electrochemicaldecaying or corrosion) or other readily condensable metal to the porousmetal emitter (e.g., on the surface at or substantially near the tip ofthe emitter). For example, a layer of zinc can be applied to a poroustungsten emitter. A surface of the tip of an emitter (e.g., an emitterin an array) can also be treated/processed by attaching carbon nanotubesto a surface of each emitter at or substantially near the tip of theemitter(s) (e.g., so that a pore size at the tip of each emitter issmaller than a pore size at a base of each emitter).

In another aspect there is a method of forming one or more emitterbodies made of porous ceramic xerogel. The method includes preparing agel solution comprising a solvent, an acidic aluminum salt, a polymer,and a proton scavenger. The method includes providing a mold for one ormore emitter bodies, each emitter body of the one or more emitter bodiescomprising a base and a tip. The method includes pouring the gelsolution into the mold. The method includes drying the gel solution inthe mold to form the one or more emitter bodies made from the porousceramic xerogel.

In some embodiments, the method can include mixing aluminum chloridehexahydrate, polyethylene oxide, water, ethanol, and propylene oxide toform the gel solution. In some embodiments, the method can includemixing 1 part by mass of polyethylene oxide, 50 parts by mass water,54.4 parts by mass ethanol, 54.4 parts by mass propylene oxide, and 54parts by mass of aluminum chloride hexahydrate to form the gel solution.In some embodiments, the method can include forming the mold from one ormore of polydimethylsiloxane (PDMS), polytetrafluoroethylene (PTFE),polymers, fluoropolymers, paraffin wax, silica, glass, aluminum, andstainless steel. In some embodiments, the porous ceramic xerogel isalumina xerorgel. In some embodiments, the porous ceramic xerogelcomprises pores approximately 3-5 μm in diameter.

In another aspect there is a method of forming one or more emitterbodies made from porous ceramic material. The method includes preparinga slurry of at least silica, water, and a ceramic component. The methodincludes providing a mold for one or more emitter bodies, each emitterbody of the one or more emitter bodies comprising a base and a tip. Themethod includes pouring the slurry into the mold. The method includesfreezing the slurry in the mold to form a frozen slurry. The methodincludes freeze drying the frozen slurry to form the one or more emitterbodies made from the porous ceramic material.

In some embodiments, the method can include low-temperature sinteringthe one or more emitter bodies. In some embodiments, the ceramiccomponent comprises alumina. In some embodiments, the slurry furthercomprises one or more of ethanol, isopropanol, and glycerol. In someembodiments, the method can include forming the mold from one or more ofpolydimethylsiloxane (PDMS), polytetrafluoroethylene (PTFE), polymers,fluoropolymers, paraffin wax, silica, glass, aluminum, and stainlesssteel. In some embodiments, the porous ceramic material comprises poresapproximately 3-50 μm in diameter. In some embodiments, the method caninclude freezing the slurry in the mold to form the frozen slurry byapplying a temperature gradient to the slurry such that the temperatureincreases from an emitter tip portion of the mold to an emitter baseportion of the mold.

In another aspect there is a method of forming one or more emitterbodies made from porous ceramic material. The method includes preparinga gel solution comprising a solvent, an acidic aluminum salt, a polymer,and a proton scavenger. The method includes drying the solution in themold to form a porous ceramic xerogel. The method includes grinding theporous ceramic xerogel to form ground porous ceramic xerogel. The methodincludes preparing a slurry of at least silica, water, and the groundporous ceramic xerogel. The method includes providing a mold for one ormore emitter bodies, each emitter body of the one or more emitter bodiescomprising a base and a tip. The method includes pouring the slurry intothe mold. The method includes freezing the slurry in the mold to form afrozen slurry. The method includes freeze drying the frozen slurry toform the one or more emitter bodies made from the porous ceramicmaterial.

In some embodiments, the method can include low-temperature sinteringthe one or more emitter bodies. In some embodiments, the slurry furthercomprises one or more of ethanol, isopropanol, and glycerol. In someembodiments, the method can include forming the mold from one or more ofpolydimethylsiloxane (PDMS), polytetrafluoroethylene (PTFE), polymers,fluoropolymers, paraffin wax, silica, glass, aluminum, and stainlesssteel. In some embodiments, the porous ceramic material comprises poresapproximately 3-50 μm in diameter. In some embodiments, the method caninclude grinding the porous ceramic xerogel into particles less thanapproximately 3-50 μm in diameter to form ground porous ceramic xerogel.In some embodiments, the method can include freezing the slurry in themold to form the frozen slurry by applying a temperature gradient to theslurry such that the temperature increases from an emitter tip portionof the mold to an emitter base portion of the mold.

Other aspects and advantages of the invention can become apparent fromthe following drawings and description, all of which illustrate theprinciples of the invention, by way of example only.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages of the invention described above, together with furtheradvantages, may be better understood by referring to the followingdescription taken in conjunction with the accompanying drawings. Thedrawings are not necessarily to scale, emphasis instead generally beingplaced upon illustrating the principles of the invention.

FIG. 1 is a schematic of Taylor Cone formation from a pressure fedcapillary emitter.

FIG. 2 is a schematic of an ion source, according to an illustrativeembodiment of the invention.

FIG. 3 shows a schematic for a method for manufacturing a porous metalelectrospray emitter, according to an illustrative embodiment of theinvention.

FIG. 4 shows a schematic of a setup for electrochemical etching,according to an illustrative embodiment of the invention.

FIG. 5 shows a schematic of a porous metal electrospray emitter array,according to an illustrative embodiment of the invention.

FIG. 6 is a drawing of a porous electrospray emitter array assembly,according to an illustrative embodiment of the invention.

FIG. 7A is a graph showing time of flight measurements for a porousmetal electrospray emitter array, according to an illustrativeembodiment of the invention.

FIG. 7B is another graph showing time of flight measurements for aporous metal electrospray emitter array, according to an illustrativeembodiment of the invention.

FIG. 8A is a graph showing thrust measurements for a porous metalelectrospray emitter array, according to an illustrative embodiment ofthe invention.

FIG. 8B is another graph showing thrust measurements for a porous metalelectrospray emitter array, according to an illustrative embodiment ofthe invention.

FIG. 9A is a graph showing current and voltage measurements for a porousmetal electrospray emitter array, according to an illustrativeembodiment of the invention.

FIG. 9B is another graph showing current and voltage measurements for aporous metal electrospray emitter array, according to an illustrativeembodiment of the invention.

FIG. 10A is a graph showing the percentage of current for a porous metalelectrospray emitter array, according to an illustrative embodiment ofthe invention.

FIG. 10B is another graph showing the percentage of current for a porousmetal electrospray emitter array, according to an illustrativeembodiment of the invention.

FIG. 11A is a graph showing the specific impulse for a porous metalelectrospray emitter array, according to an illustrative embodiment ofthe invention.

FIG. 11B is another graph showing the specific impulse for a porousmetal electrospray emitter array, according to an illustrativeembodiment of the invention.

FIG. 12 is a schematic of an ion source, according to an illustrativeembodiment of the invention.

FIGS. 13A and 13B are schematics of an ion source, according to anillustrative embodiment of the invention.

FIG. 14 is a schematic of an electrospray emitter array, according to anillustrative embodiment of the invention.

FIG. 15 is a schematic of an electrospray emitter array, according to anillustrative embodiment of the invention.

FIG. 16 shows a schematic for a method for manufacturing an electrosprayemitter, according to an illustrative embodiment of the invention.

FIG. 17 shows a schematic for a method for manufacturing an electrosprayemitter, according to an illustrative embodiment of the invention.

FIG. 18 is a flowchart illustrating a method for manufacturingelectrospray emitters.

FIG. 19 is a flowchart illustrating a method for manufacturingelectrospray emitters.

FIG. 20 is a flowchart illustrating a method for manufacturingelectrospray emitters.

FIG. 21 is a schematic of a mold for emitter bodies.

FIG. 22 is a graph showing the relative variation of the emitter voltageimmediately after polarity reversal.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is a schematic of an ion source 100, according to an illustrativeembodiment of the invention. The ion source 100 includes a body 105(e.g., an emitter body) that includes a base 110 and a tip 115. The body105 can be made of a porous metal (e.g., a microfabricated emitter bodyformed from a porous metal substrate) compatible with an ionic liquid ora room temperature molten salt (e.g., does not react or result inelectrochemical decaying or corrosion). The body 105 can be mountedrelative to a source 120 of ionic liquid or a source of a roomtemperature molten salt. The body 105 includes a pore size gradient thatdecreases from the base 110 of the body 105 to the tip 115 of the body105, such that ionic liquid can be transported through capillarity(e.g., through capillary forces) from the base 110 to the tip 115. Theionic liquid can be continuously transported through capillarity fromthe base 110 to the tip 115 so that the ion source 100 (e.g., emitter)avoids liquid starvation. An electrode 125 can be positioned downstreamrelative to the body 105. A power source 130 can apply a voltage to thebody 105 relative to the electrode 125, thereby emitting a current(e.g., a beam of ions 135) from the tip 115 of the body 105. Theapplication of a voltage can cause formation of a Taylor cone (e.g., asshown in FIG. 1) at the tip 115 and cause the emission of ions 135 fromthe tip 115.

In some embodiments, the body 105 is an emitter that is a cylindricalneedle or a flat ribbon-like needle. Emitter geometry (e.g., shapeand/or configuration of the emitter body) can affect the currentgenerated by the emitter. For instance, flat ribbon-like configurationsyield more current than traditional cylindrical solid needles. Atungsten externally wetted emitter can generate about 0.2 μA peremitter. In contrast, a flat ribbon tungsten emitter can generate up toabout 10 μA per emitter. In some embodiments, a radius of curvature ofthe tip 115 of the body 105 can be in the range of about 1 μm to about20 μm in the horizontal direction (e.g., along the z axis) and a radiusof curvature of about 2 μm to about 3 μm in the vertical direction(e.g., along the y axis).

The body 105 can be microfabricated from a porous metal substrate. Body105 can be formed by electrochemical etching. In some embodiments, thebody can be formed of a porous metal substrate (e.g., tungsten) butother materials may be present. The body 105 can be microfabricated froma porous metal compatible (e.g., does not react or result inelectrochemical decaying or corrosion) with ionic liquids and/or roomtemperature molten salts. Examples of such porous metals includetungsten, nickel, magnesium, molybdenum, or titanium.

The pore size gradient of the body 105 can allow ionic liquid from thesource 120 to be transported from the base 110 to the tip 115. In someembodiments, the size of the pores in the base 110 are larger than thepores in the metal at the tip 115, which allows for the ionic liquid tobe transported through capillarity (e.g., capillary forces) from thebase 110 of the emitter to the tip 115. By transporting the ionic liquidthrough capillarity, the pore size gradient can act as a passive,self-regulating capillary supply that reduces mass and complexity overcapillary systems (e.g., by substantially reducing the need for apressurized system). The pore size gradient can continuously providesionic liquid to the tip 115, reducing the chances that the ion sourcewill suffer from liquid starvation. Flow throughout the body (e.g.,increased volume flow from the pores) can allow for even more currentthan solid ribbon emitters.

FIG. 2 depicts an ion source comprising an emitter body 105, however, aplurality of emitters (e.g., an array of emitters) can be used in a 1Dor 2D array. The array of emitters can also be microfabricated from aporous metal (e.g., a unitary porous metal substrate) compatible withthe at least one of ionic liquid or room-temperature molten salt. Eachemitter, as described above, can have a pore size gradient thatdecreases from the base of the emitter to the tip of the emitter so thatthe ionic liquid is transported through capillarity from the base to thetip of each emitter. An electrode (e.g., electrode 125) can bepositioned downstream relative to the array of emitters and a powersource (e.g., power source 130) can provide a voltage to the array ofemitters with respect to the electrode.

FIG. 3 shows a schematic for a method for manufacturing a porous metalelectrospray emitter, according to an illustrative embodiment of theinvention. Single emitters (e.g., emitter body 105 as shown in FIG. 2)or arrays of emitters (e.g., 1D or 2D arrays) can be manufactured fromporous metallic substrates using micro fabrication techniques, such asphotolithography and electrochemical etching. Porous metal emitter(s)can be microfabricated using electrochemical etching with a polyimidefilm as a masking layer. A method for manufacturing the emitters caninclude the following steps: (1) filling a porous substrate withpositive photoresist (Step 300), (2) applying a layer of polyimide tothe sample (e.g., the porous substrate with the photoresist) (Step 310),(3) applying a layer of positive photoresist on the polyimide andexposing the photoresist to transfer the intended geometry (Step 320),(4) developing the photoresist (e.g., to remove the exposed photoresist)and etching the polyimide (Step 330), (5) removing the photoresist(e.g., leaving only the polyimide mask defined by the intended emittergeometry) (Step 340), and (6) etching the sample to form theemitter/emitter arrays (Step 350). The method can also includeprocessing and/or treating a tip of each emitter to vary a pore sizebetween the base and the tip of the emitter.

A method for manufacturing the emitters can include the step ofproviding a porous metal substrate 360. The emitter body can be formedfrom a porous metal compatible (e.g., does not react or result inelectrochemical decaying or corrosion) with an ionic liquid or roomtemperature molten salt. For example, tungsten sheets (e.g., poroustungsten sheets with a 0.25 mm thickness and 2 micron porosity fromAmerican Elements, Los Angeles, Calif.) can be cut into 1 cm by 2.5 cmpieces using a diesaw (e.g., Disco Abrasive System Model DAD-2H/6T fromDISCO, Tokyo, Japan) and cleaned in acetone followed by isopropanol.Other porous metals compatible with ionic liquids and room temperaturemolten salts can be used as well. For example, the porous metal can benickel, magnesium, molybdenum, titanium, or any combination thereof. Insome embodiments, a unitary substrate of a porous metal can be used toform more than one emitter (e.g., an emitter geometry pattern that canbe used to form an emitter array). The use of porous metal results inthe increased capillary flow capacity provided by the volumetricporosity of the emitter substrate. The emitters can be manufactured fromone or more substrates to form one or more flat emitters (e.g.,needles).

The porous metal substrate can be developed to form a sample thatincludes porous metal substrate 360 (e.g., porous tungsten substrate)with the pores blocked (Step 300), for example, with a photoresist 370.The porous metal substrate can be filled with photoresist 370 (e.g.,Shipley 1827 positive photoresist) and the substrate exposed (e.g., bothsides) with UV light to block pores of the porous metal to form thesample. In some embodiments, the substrate can be allowed to soak in thephotoresist 370 (e.g., for 20 seconds). The sample (e.g., the porousmetal substrate with the photoresist) can be spun for 60 secondsstarting at 700 rpm and increased to 1700 rpm with an acceleration of200 rpm/s. The sample can then be baked by heating on a hotplate for 20seconds at 70° C. followed by 30 seconds in an oven at 90° and 30seconds at 130°. In some embodiments, both sides of the sample can beexposed using a broadband aligner (e.g., a Karl Suss MJB3 fromSuss-MicroTec, Waterbury Center, Conn.) for 150 seconds and immersed ina developer (e.g., a high pH solution developer such as MF-319 to “wash”away the material to be eliminated upon UV exposure) until both surfacesare cleared of photoresist. A broadband aligner can be used inmicrofabrication to transfer a pattern to a photoresist-coated substrateby shining UV light. If no pattern is to be applied, the UV light can beused to produce the decay on exposed surfaces. The photoresist 370 canbe left substantially filling the bulk of the porous media to preventpolyimide from entering the pores. The samples can be cleaned indeionized (DI) water and dried.

In some embodiments, the surface of the porous metal 360 can be blockedby the uniform deposition of mono-layers of a compatible metal usingChemical Vapor Deposition (CVD) (e.g., thermal evaporation techniqueinvolving boiling and depositing a material on to a relatively coldersurface, such as, for example, depositing the compatible metal on to theporous substrate). Mono-layers of a compatible metal using CVD can bedeposited instead of soaking the porous metal substrate in photoresistpolymers. One benefit is that pores can be substantially clear ofpotential contamination that could hurt the etching process andelectrospray operation. A “compatible metal” can be a metal that iscompatible with ionic liquids and the porous substrate material (e.g.,does not react or result in electrochemical decaying or corrosion). Forexample, if the porous metal substrate 360 is a porous tungstensubstrate, the surface of the porous tungsten substrate can be blockedby the uniform deposition of mono-layers of tungsten using CVD.

The method can also include the step of adding a layer of polyimide 380(e.g., PI2556 polyimide from HD Microsystems) to a first side of thesample (e.g., the porous metal substrate 360 with the pores blocked withthe photoresist 370) (Step 310). The sample can be prebaked to drive offsolvents. Polyimide 380 can be used as the masking material for itsresistance to Sodium Hydroxide and ability to be precisely patternedusing standard photolithography techniques. In some embodiments, a 1.5μm layer of polyimide 380 is spun onto one surface of the sample. Thesample can be prebaked by pooling the polyimide 380 on the surface for10 seconds, spun at 500 rpm for 5 seconds and slowly ramped up to 1300rpm and spun for 50 seconds. The polyimide 380 can be heated on ahotplate at 55° for 30 seconds and 70° for 30 seconds followed by ovenbakes at 90° for 60 seconds and 130° for 60 seconds. The gradual heatingprotocol employed can limit the amount of holes in the polyimide 380caused by gas trapped in the bulk of the porous media escaping duringrapid heating.

The method can also include the step of applying photoresist 370 to thefirst side of the sample (e.g., porous metal substrate with the blockedpores and including a layer of polyimide) (Step 320). A layer ofphotoresist 370 can be applied on top of the polyimide 380 (e.g., thelayer of polyimide 380 applied to the porous metal substrate 360 in Step310 above). In some embodiments, a layer of photoresist 370 having athickness of about 5 μm is spun onto the polyimide 380. The sample canbe heated at 70° for 30 seconds on a hotplate and 130° for 90 seconds inan oven. A transparency mask (e.g., photolithography transparencies fromPageWorks, Cambridge, Mass.) can be applied to the first side of thesample and exposed with a UV light to form the emitter geometry pattern390 and exposed parts of positive photoresist 400.

The sample can be developed to transfer the emitter geometry pattern 390to the sample. The sample can be developed to remove the exposedpositive photoresist 400 from the sample (Step 330). The exposed partsof the positive photoresist 400 can be removed to etch the underlyingpolyimide 380 (e.g., to remove the portion of the polyimide 380 coveredby exposed parts of the photoresist and leave the portion of thepolyimide covered by the unexposed photoresist), thereby transferringthe desired emitter geometry pattern 390. In some embodiments, samplesare exposed for 120 seconds and developed in MF-319 until the pattern istransferred to the polyimide.

The method can also include the step of cleaning photoresist off thesample (e.g., cleaning off the layer of the unexposed photoresist fromStep 320) and then curing the sample (Step 340). Curing the polyimide380 (e.g., the remaining polyimide defined by the emitter geometrypattern 390) in an oven hardens the polyimide 380 against theelectrochemical etch chemistry. The samples can be immersed in acetonefor 1 hour in an ultrasonic cleaner to remove the photoresist 370 fromthe surface and the bulk (e.g., to remove the photoresist 370 that wasunexposed from Step 320 and also the photoresist 370 that filled thepores of the porous metal substrate 360 in Step 300). The samples can bebaked in an anneal furnace to partially cure the polyimide 380 using thefollowing temperature profile: a slow ramp rate from room temperature to150° C., hold at 150° for 10 minutes then ramp up to 200° and hold for10 minutes in nitrogen, then a ramp up to 240° and hold for 1.5 hours innitrogen followed by a slow cool down period.

The sample can be then electrochemically etched to form the emittergeometry 390 (Step 350). The sample (e.g., a unitary substrate of porousmetal that has undergone the Steps 300-340 above) can be etched toremove excess porous metal 360 to form the desired emitter geometry 390(e.g., one or more emitters where each emitter has a base at the firstend of the substrate and a tip at the second end of the substrate). Thesample can be etched, for example, in Sodium Hydroxide until the excessporous metal 360 (e.g., excess tungsten or other metal) is removed toshape the emitter(s) according to a desired geometry. The emitter(s) canbe microfabricated by etching the sample to remove excess porous metalto form, for example, one or more conical shaped emitters or a one ormore flat ribbon-like needles/emitters. The remaining polyimide 380 canbe then removed, thereby providing the porous metal emitter array withthe desired emitter geometry pattern 390.

A tip of an emitter, or an individual emitter in an emitter array, canbe processed and/or treated to vary a pore size between the base of theemitter body to a tip of the emitter body. In some embodiments, thesmallest pores (e.g., relative to the other pores in the emitter) arenear the emitter tips. The emitter body can be manufactured to have apore size gradient that decreases from a first end of the body to asecond end of the body (e.g., the pore size becomes smaller towards thesecond end/tip of the body, so that the sizes of the pores at the secondend/tip are smaller than the pore size at the first end/base of thebody). The pore size gradient allows the ionic liquid to be continuouslytransported through capillarity from the first end of the body to thesecond of the body (e.g., from the base of the emitter to the tip of theemitter).

A nano/meso porous layer of a compatible, electrically conductivematerial (e.g., zinc on porous tungsten) can be applied to the surfacesubstantially near/around the tip of each emitter to vary a pore size(e.g., to form smaller pores at the tip relative to the base). The sizeof the pores in the layer of the compatible material can besubstantially smaller than the size of the pores in the porous substrate(e.g., the porous emitter). A “compatible metal” can be a metal that iscompatible with ionic liquids and the porous substrate material (e.g.,does not react or result in electrochemical decaying or corrosion). Acompatible metal (e.g., zinc for porous tungsten) can be depositedthrough thermal evaporation. The compatible metal (e.g., zinc) can formaggregates over the porous metal emitter (e.g., porous tungsten). Insome embodiments, a layer about 1-5 microns thick of compatible metalcan be deposited. Carbon nanotubes can also be attached to the surfaceof the emitter substantially near/around tip of each emitter to form thepore size gradient (e.g., to form smaller pores at the tip relative tothe base). Carbon nanotubes can be deposited on the surface (e.g., at orsubstantially near the tip of the emitter) forming a relativelywell-organized porous “forest.” In both cases, the introduction ofdissimilar porosities for preferential flow (e.g., pore sizes smaller atthe tip than the base of the porous metal emitter) facilitates liquidtransport to the emission sites (e.g., the tip of the emitter).

Traditional ion sources using normal solvents (with non-zero vaporpressures) do not, in principle, use controlled pore variation since theliquid/vapor interface is in equilibrium (e.g., water with water vapor)and is convected outwards through evaporation, including inside thepores. Porous metal electrospray emitters, however, have no preferentialdirection for convection since there is no thermal evaporation fromionic liquids. There is only ion evaporation, but for it to occur, theliquid is transported to the tips through capillarity (e.g., capillaryforces).

FIG. 4 shows a schematic of a setup 401 for electrochemical etching,according to an illustrative embodiment of the invention. As notedabove, emitters can be manufactured from electrochemically etched porousmetal substrates (e.g., porous tungsten) with a polyimide layer actingas an etch mask. Isotropic etching (e.g., step 350 in FIG. 3) can beperformed to form emitter geometry. The masked sample 410 (e.g., thesample from Step 340) can be placed into a container filled with anetchant solution 420 (e.g., IN sodium hydroxide (NaOH)). An electricpotential (e.g., DC electric potential) can be applied using a powersource 429 between the sample 410 and a cathode 430 (e.g., a stainlesssteel cathode) to initiate the etching process. The etching can beperformed, for example, in a glass beaker 440 with a circular cathodesurrounding the piece. To aid in the formation of even tips and toenhance the etching rate, the porous metal sample 410 can be removedperiodically and immersed in an ultrasonic cleaner to clear the surfaceof the residue and to remove bubbles that form on the surface. Theetching can also be carried out in a uniform flow of etchant, which canreduce the effect of eddies and bubble formation on the etch. Followingthe completion of the etch, sodium hydroxide can be rinsed off thesample in DI water. The remaining polyimide mask (e.g., the remainingpolyimide mask defining the emitter geometry as described in Steps 340and 350 in FIG. 3 above) can be removed in Piranha (e.g., 4:1 mixture ofsulfuric acid and hydrogen peroxide). Following a rinse in DI water, theemitters can be blown dry with nitrogen.

FIG. 5 shows a schematic of a porous metal electrospray emitter array500, according to an illustrative embodiment of the invention. Theemitters 510A-510E can be manufactured to have emitter spacing 520 ofless than about 1 mm. Each emitter can have a pore size gradient thatdecreases from the base 530A-530E of the emitter to the tip 540A-540E ofthe emitter 510A-510E. The ionic liquid can be continuously transportedthrough capillarity (e.g., capillary forces) from the base 530A-530E tothe tip 540A-540E of each emitter 510A-510E. In some embodiments, anyone of emitter(s) 510A-510E can have a radius of curvature of about 10μm to about 20 μm in the horizontal direction (e.g., along the y-axis)and a radius of curvature of about 2 μm to about 3 μm in the verticaldirection (e.g., along the x-axis). The emitter array 500 can include aplurality of emitters 510A-510E microfabricated from a porous metal. Theporous metal can be compatible (e.g., does not react or result inelectrochemical decaying or corrosion) with an ionic liquid orroom-temperature molten salt. The emitters can be formed byelectrochemical etching. In some embodiments, the porous metal istungsten, nickel, magnesium, molybdenum, titanium, or any combinationthereof.

FIG. 6 is a drawing of a porous metal electrospray emitter arrayassembly 600, according to another illustrative embodiment of theinvention. A plurality of emitters (e.g., two or more emitters) can begrouped together to form a thruster. The typical thrust of a singleemitter/needle operating in the ionic mode is on the order of about 0.05μN/μA to about 0.1 μN/μA, therefore emitters can be grouped to produceas much current as possible (e.g., for different space applications). Athruster assembly can include emitter sheets 620A-620C (e.g., where eachsheet includes one or more one emitters) together to create a 2D arrayof emitters. The emitters can be placed relative to an extractor 630,resulting in the generation of an ionic beam.

In this embodiment, the ion source 600 is a thruster that includes a setof 3 flat needle arrays 620A-620C, each containing up to 18 individualemitters giving a maximum of 54 emitters with a tip to tip separation ofabout 1 mm. The thruster 600 provided an emitter density of a littleunder 0.5 tips per mm². In this embodiment, individual emitter sheets620A-620C are clamped in place between two bars 640 (50×7.9×7.9 mmstainless steel). Emitter sheet separation can be provided by plates 641(e.g., 1.5 mm inch thick stainless steel plates 165 cut by a waterjet).

Extractor 630 can be made from stainless steel (e.g., a 0.635 mm thickstainless steel sheet). In this embodiment, the individual extractorslits 650 are 1 mm wide which gives clearance for a beam spreading halfangle of 51 degrees when the emitter tips are just touching theextractor slit plane. The extractor 630 can be attached to the holderbars 670 using fastening mechanism 660 and 670 (e.g., two polycarbonate#6-32 screws with two polyethylene spacers). The combination of screws660 (e.g., or other similar fastening device) and spacers 670 canprovide electrical insulation between the extractor 630 and emitters620A-620C and can inhibit the liquid fuel from migrating to theextractor and causing a short.

The thruster assembly 600 can provide precise alignment between theemitter sheets 620A-620C and the extractor grid 630 to reduce beamimpingement. The thruster assembly 600 can provide adequate insulation(e.g., electrical and fluidic insulation) between the extractor 630 andemitters 620A-620C to reduce the risk of electrical shorting. Theassembly 600 includes materials that are compatible (e.g., does notreact or result in electrochemical decaying or corrosion) with ionicliquids for long periods of time. The thruster assembly 600 is easy toassemble, to reduce the risk of breaking emitters.

FIGS. 7A-7B show graphs 700 and 710 show time of flight measurements fora porous metal electrospray emitter array. Plots 700 and 710 show thenormalized intensity of a beam of ions generated by a porous metalElectrospray emitter array, as a function of time. The emitters testedused EMI-BF4 (3-ethyl-1-methylimidazolium tetrafluoroborate) as theIonic Liquid. EMI-IM (1-ethyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide) can also be used as the ionic liquid.The time-of-flight technique was used to determine the composition ofthe emitted beam on a single flat needle array containing 6 emitters.The time of flight curves for positive (1915.28 V) and negative(−1898.33 V) emission are shown in Graphs 700 and 710, respectively.Time-of-flight mass spectrometry shows that the emitted beam is purelyionic and includes two species of ions in both positive and negativemodes of operation. The contribution of each ion species to the totalcurrent can be calculated by looking at the relative changes in measuredcurrent during the time-of-flight tests. The lack of an elongated tailfollowing the steps indicates that there are no droplets contributing tothe emitted current. Operating an electrospray source in a mixeddroplet-ion regime could be very costly in terms of efficiency andspecific impulse.

In FIGS. 7A-7B, the time it takes for particles to travel a knowndistance was measured and then a charge to mass ratio was calculatedbased on the velocity. From the drift time measured for the particles totravel between the given distance between the electrostatic gate and thedetector (L=751.57 mm) the specific charge ratio and subsequently themass of the species can be calculated using EQN 1.

$\begin{matrix}{\left( \frac{q}{m} \right)_{1} = \frac{\left( {L/t_{1}} \right)^{2}}{2\varphi_{B}}} & {{EQN}.\mspace{11mu} 1}\end{matrix}$

where “L” is the drift distance in the time-of-flight spectrometer, “m”is the particle mass, “q” is the particle electric charge, “t₁” is thetime of flight over drift distance, and “ΦB” is the on-axis acceleratingpotential. As an approximation, the on-axis accelerating potential canbe taken to be equal to the extraction potential. In reality, it can beup to 7 eV lower. The results are tabulated in table 1.

TABLE 1 Emitted Beam Composition Polarity Time of Flight (μs) Mass (amu)% of Total Current $\left( \frac{q}{m} \right)C\text{/}{gr}$Correponding Ion Positive 13.9 112.40 42.58 430.6 [EMI]⁺  (1915.28 V)22.8 310.15 57.42 [EMI-BF₄][EMI]⁺ Negative 12.4 89.49 49.13 513.9 [BF⁻](-1898.33) 21.9 287.28 50.87 [EMI-BF₄][BF⁻]

FIGS. 8A-8B are graphs 800 and 810 showing thrust measurements for theporous metal electrospray emitter array containing 6 emitters from FIGS.7A-7B. Graphs 800 and 810 show the measured thrust as a function ofextraction potential for a negative extraction voltage range and apositive extraction voltage range, respectively. Thrust measurementswere conducted at the Busek Company, Natick, Mass. using a torsionalbalance capable of measuring sub micro newton forces. Porous metalelectrospray emitters have been shown to support an increase in currentof over an order of magnitude as compared to solid cylindrical emitters.The results show that the thruster produced from about 0.82 μN to about2.33 μN in the −1282 V to −2088 V negative extraction voltage range 800and from about 1.08 μN to about 5.67 μN in the 1391 V to 2437 V positiveextraction voltage range 810. This corresponds to a thrust per emittertip of about 0.048 μN at −2088 V and about 0.116 μN at 2437 V. Theleveling off of thrust in the negative mode can be due to the thrusterapproaching the limit of its ability to transport liquid to the tip.

FIGS. 9A-9B show current as a function of extraction voltagemeasurements for the porous metal electrospray emitter array containing6 emitters from FIGS. 7A-7B. Graphs 900 and 910 show the measuredcurrent in a beam of ions generated by the porous metal Electrosprayemitter array as a function of extraction voltage for a negativeextraction voltage range and a positive extraction voltage range,respectively. Plot 940 is the current measured/lost in the extractor(e.g., electrode), plot 930 is the current measured in the beam of ionsand plot 920 is the total current collected. FIGS. 10A-10B show thepercentage of current for the same emitter array. Graphs 1000 and 1010show the percentage of total current lost to the extractor as a functionof extractor voltage for a negative extraction voltage range and apositive extraction voltage range, respectively. A small fraction of thebeam current (about 10%-20%) was lost to the extractor which is due tobeam impingement. The extractor geometry can be changed aligned tominimize the current lost to the extractor. Extractors can be, forexample, positioned a distance about 1 emitter height away from the tipand can be aligned through the fabrication of alignment features on thesubstrates. The extractor thickness can also be reduced to minimizecurrent lost to the extractor.

The beam current can be extracted using the following equation:

$\begin{matrix}{\frac{F}{I_{B}} = \sqrt{\frac{2\varphi_{B}}{\left( {q/m} \right)}}} & {{EQN}.\mspace{14mu} 2}\end{matrix}$

where F is the measured thrust and I_(B) is the Electrospray beamcurrent. The specific charge ratio can be calculated as described abovein EQN. 1 for the time of flight measurements. In addition, there canexist some beam current unaccounted for in the extractor current andcollected current, due to the effect of secondary electrons caused bythe high energy ions hitting the extractor and the collector. Thecollector can be biased to trap the secondaries and reduce this effect.

FIGS. 11A-11B show the specific impulse for the porous metalelectrospray emitter array containing 6 emitters from FIGS. 7A-7B.Graphs 1100 and 1110 show the specific impulse measured as a function ofextraction voltage for a positive extraction voltage range and anegative extraction voltage range, respectively. Plot 1120 charts themeasured specific impulse and plot 1130 charts the maximum specificimpulse. In the negative polarity regime 1110, the emitters produced upto about −57.17 μA in current and a thrust of up to about 2.33 μN,yielding a specific impulse (ISP) of about ISP of about 2000 to 3000seconds. In the positive polarity regime 1100, the emitters produced upto about 69.84 μA in current and a thrust of up to about 5.67 μN,yielding an ISP of about 3000 to 5000 seconds.

In some embodiments of the invention, emitters and/or emitter arrays canbe fabricated from a dielectric material (e.g., a ceramic, glass, orother oxide material). In some embodiments, emitters and/or emitterarrays can be fabricated from a metal material (e.g., silver, stainlesssteel, tungsten, nickel, magnesium, molybdenum, titanium, anycombination thereof, or any of these metals coated with a noble metalmaterial such as platinum or gold). Some embodiments of the inventioninclude a distal electrode, as described in greater detail below. Insome embodiments, the distal electrode can be made from a metal material(e.g., silver, stainless steel, tungsten, nickel, magnesium, molybdenum,titanium, any combination thereof, or any of these metals coated with anoble metal material such as platinum or gold).

FIG. 12 is a schematic of an ion source 1200, according to anillustrative embodiment of the invention. The ion source 1200 includes abody 1205 (e.g., an emitter body) that includes a base 1210 and a tip1215. The body 1205 can be made of a porous material (e.g., amicrofabricated emitter body formed from a porous material substrate)compatible with an ionic liquid or a room temperature molten salt. Thebody 1205 can be externally wetted by an ionic liquid or a roomtemperature molten salt (e.g., ionic liquid 1220). The body 1205includes a pore size gradient that decreases from the base 1210 of thebody 1205 to the tip 1215 of the body 1205, such that ionic liquid 1220can be transported through capillarity (e.g., through capillary forces)from the base 1210 to the tip 1215. For example, ionic liquid 1220 canbe continuously transported through capillarity to the tip 1215 so thatthe ion source 1200 (e.g., emitter) avoids liquid starvation. Anextractor electrode 1225 can be positioned downstream relative to thebody 1205. A power source 1230 can apply a voltage to ionic liquid 1220relative to the extractor electrode 1225, thereby emitting a currentfrom the tip 1215 of the body 1205. The application of a voltage cancause formation of a Taylor cone (e.g., as shown in FIG. 1) at the tip1215 and cause the emission of ions from the tip 1215. In theillustrated embodiment, the ionic liquid 1220 can be the distalelectrode.

In some embodiments, the body 1205 is an emitter that is a cylindricalneedle or a flat ribbon-like needle. As noted above, emitter geometry(e.g., shape and/or configuration of the emitter body) can affect thecurrent generated by the emitter. For instance, flat ribbon-likeconfigurations yield more current than traditional cylindrical solidneedles. The body 1205 can be microfabricated from a porous material. Insome embodiments, the body 1205 can be formed of a porous metalsubstrate but other materials may be present. The body 1205 can bemicrofabricated from a porous metal compatible with ionic liquids and/orroom temperature molten salts. Examples of such porous metals includesilver, stainless steel, tungsten, nickel, magnesium, molybdenum,titanium, any combination thereof, or any of these metals coated with anoble metal material such as platinum or gold. The body 1205 can bemicrofabricated from a porous ceramic, porous glass, or other porousoxide material compatible with ionic liquids and/or room temperaturemolten salts.

The pore size gradient of the body 1205 can allow ionic liquid 1220 tobe transported to the tip 115. In some embodiments, the size of thepores in the base 1210 are larger than the pores in the tip 1215, whichallows for the ionic liquid to be transported through capillarity (e.g.,capillary forces) from the base 1210 of the emitter to the tip 1215. Bytransporting the ionic liquid through capillarity, the pore sizegradient can act as a passive, self-regulating capillary supply thatreduces mass and complexity over capillary systems (e.g., bysubstantially reducing the need for a pressurized system). The pore sizegradient can continuously provides ionic liquid to the tip 1215,reducing the chances that the ion source 1200 will suffer from liquidstarvation. Flow throughout the body (e.g., increased volume flow fromthe pores) can allow for even more current than solid emitters.

In some embodiments, the body can be made from a porous dielectricmaterial. Beneficially, electric forces, in addition to the capillaritydescribed above, can facilitate directing the ionic liquid to the body'stip because the dielectric material does not shield the ionic liquid asa body made from metal. In some embodiments, the body can be made from aporous dielectric material without a pore size gradient, where electricforces can facilitate directing the ionic liquid to the body's tip.

FIGS. 13A and 13B are schematics of an ion source 1300, according to anillustrative embodiment of the invention. FIG. 13A depicts a side crosssection of the ion source 1300. FIG. 13B depicts a top view of the ionsource 1300. The ion source 1300 includes a body 1305 (e.g., an emitterbody) that includes a base 1310 and a tip 1315. The body 1305 can bemade of a porous material (e.g., a microfabricated emitter body formedfrom a porous material substrate) compatible with an ionic liquid or aroom temperature molten salt. The body 1305 is disposed in ionic liquid1320. Ionic liquid 1320 is surrounded by distal electrode 1322. The body1305 includes a pore size gradient that decreases from the base 1310 ofthe body 1305 to the tip 1315 of the body 1305, such that ionic liquid1320 can be transported through capillarity (e.g., through capillaryforces) to the tip 1315. For example, ionic liquid 1320 can becontinuously transported through capillarity to the tip 1315 so that theion source 1300 (e.g., emitter) avoids liquid starvation. An extractorelectrode 1325 can be positioned downstream relative to the body 1305. Apower source 1330 in electrical connection with distal electrode 1322can apply a voltage to ionic liquid 1320 relative to the extractorelectrode 1325, thereby emitting a current from the tip 1315 of the body1305. The application of a voltage can cause formation of a Taylor cone(e.g., as shown in FIG. 1) at the tip 1315 and cause the emission ofions from the tip 1315.

In some embodiments, the body 1305 is an emitter that is a cylindricalneedle or a flat ribbon-like needle. As noted above, emitter geometry(e.g., shape and/or configuration of the emitter body) can affect thecurrent generated by the emitter. The body 1305 can be microfabricatedfrom a porous material. In some embodiments, the body 1305 can be formedof a porous metal substrate (e.g., tungsten) but other materials may bepresent. The body 1305 can be microfabricated from a porous metalcompatible with ionic liquids and/or room temperature molten salts.Examples of such porous metals include silver, stainless steel,tungsten, nickel, magnesium, molybdenum, titanium, any combinationthereof, or any of these metals coated with a noble metal material suchas platinum or gold. The body 1205 can be microfabricated from a porousceramic, porous glass, or other porous oxide material compatible withionic liquids and/or room temperature molten salts.

FIG. 14 is a schematic of an electrospray emitter array 1400, accordingto an illustrative embodiment of the invention. In this embodiment, theion source 1400 includes needle emitter array 1405. The needle emitterarray 1405 can be made of a porous material (e.g., a microfabricatedemitter body formed from a porous material substrate) compatible with anionic liquid or a room temperature molten salt. The needle emitter array1405 is disposed on ionic liquid 1420. Ionic liquid 1420 surroundsinsulator 1422 and is in contact with distal electrode 1424. Asdescribed above, the needle emitter array 1405 includes a pore sizegradient that decreases from the base to the tips of the needle emitterarray 1405, such that ionic liquid 1420 can be transported throughcapillarity (e.g., through capillary forces) to the tips. An extractorelectrode 1425 can be positioned downstream relative to needle emitterarray 1405. A power source 1430 in electrical connection with distalelectrode 1424 can apply a voltage to ionic liquid 1420 relative to theextractor electrode 1425, thereby emitting a current from the tips ofthe needle emitter array 1405.

The needle emitter array 1405 can be microfabricated from a porousmaterial. In some embodiments, the needle emitter array 1405 can beformed of a porous metal substrate (e.g., tungsten) but other materialsmay be present. The needle emitter array 1405 can be microfabricatedfrom a porous metal compatible with ionic liquids and/or roomtemperature molten salts. Examples of such porous metals include silver,stainless steel, tungsten, nickel, magnesium, molybdenum, titanium, anycombination thereof, or any of these metals coated with a noble metalmaterial such as platinum or gold. The needle emitter array 1405 can bemicrofabricated from a porous ceramic, porous glass, or other porousoxide material compatible with ionic liquids and/or room temperaturemolten salts.

FIG. 15 is a schematic of an electrospray emitter array 1500, accordingto an illustrative embodiment of the invention. The ion source 1500includes emitter array 1505. The emitter array 1505 is housed in athruster package 1510 (made from e.g., silicon). The emitter array 1505is disposed against a porous plug/isolation valve 1515. The porousplug/isolation valve 1515 is disposed against a distal electrode 1520.The porous plug/isolation valve 1515 can serve as a connecting memberbetween the emitter array 1505 and the distal electrode 1520. The distalelectrode 1520 is in contact with ionic liquid 1525. In someembodiments, each of the emitter array 1505, the porous plug/isolationvalve 1515, and distal electrode 1520 can include a pore size gradientthat decreases from its base to its top (e.g., in the direction towardthe tips of needle emitter array 1505), such that ionic liquid can betransported through capillarity. In some embodiments, the emitter array1505 can have smaller-sized pores than the porous plug/isolation valve1515, which in turn can have smaller-sized pores than the distalelectrode 1520, creating a pore size gradient that decreases from thedistal electrode 1520 to the emitter array 1505, such that ionic liquid1525 can be transported through capillarity from the distal electrode1520 to the emitter array 1505.

An extractor electrode 1530 can be positioned downstream relative to theemitter array 1505. An accelerator grid 1535 can be position downstreamrelative to the extractor grid 1530. A bipolar power source 1545 canapply a voltage to the distal electrode 1520 relative to the extractorelectrode 1530, thereby emitting a current (e.g., a beam of ions) fromthe tips of emitter array 1505. The application of a voltage can causeformation of a Taylor cone (e.g., as shown in FIG. 1) at the tips 115and cause the emission of ions from the emitter array 1505.

In some embodiments, the emitter array 1505 can be fabricated from adielectric material (e.g., a ceramic, glass, or other oxide material).In some embodiments, the emitter array 1505 can be fabricated from ametal material (e.g., silver, stainless steel, tungsten, nickel,magnesium, molybdenum, titanium, any combination thereof, or any ofthese metals coated with a noble metal material such as platinum orgold). In some embodiments, the distal electrode 1520 can be made from ametal material (e.g., silver, stainless steel, tungsten, nickel,magnesium, molybdenum, titanium, any combination thereof, or any ofthese metals coated with a noble metal material such as platinum orgold). In some embodiments, the extractor electrode 1530 can befabricated from silicon or tungsten. In some embodiments, the porousplug/isolation valve 1515 can be fabricated from a dielectric material.In some embodiments, the porous plug/isolation valve 1515 can serve as aplug or valve to prevent ionic liquid 1525 from absorbing water and/orother gases from the environment. For example, the electrospray emitterarray 1500 can be used as a thruster. The porous plug/isolation valve1515 can be opened once the thruster is in space to preventcontamination of the ionic liquid.

FIG. 16 shows a schematic for a method for manufacturing an electrosprayemitter, according to an illustrative embodiment of the invention.Single emitters or arrays of emitters (e.g., 1D or 2D arrays) can bemanufactured from porous substrates using micro fabrication techniques.Porous emitter(s) can be microfabricated by removing material from asubstrate made from a porous dielectric material (e.g., porous ceramic,glass or other oxide material) via chemical wet etching, plasma dryetching, ion beam milling, laser milling, etc. A method formanufacturing the emitters can include the following steps: (1)providing a porous material 1650 (step 1600) and (2) selectivelyremoving material from porous material 1650 (step 1710). In someembodiments, hydrolic acid can be used to open up the pores of thematerial after, e.g., the milling process.

FIG. 17 shows a schematic for a method for manufacturing an electrosprayemitter, according to an illustrative embodiment of the invention.Single emitters or arrays of emitters (e.g., 1D or 2D arrays) can bemanufactured from porous substrates using micro fabrication techniques.Porous emitter(s) can be microfabricated by depositing a porousdielectric material (e.g., porous ceramic, glass or other oxidematerial) onto a porous substrate via chemical vapor deposition,physical vapor deposition, deposition of nano-beads, etc. A method formanufacturing the emitters can include the following steps: (1)providing a porous material 1750 (step 1700) and (2) selectivelydepositing porous dielectric materials 1760 onto the porous material1750 (step 1710). In some embodiments, a mold can be used to form theemitters (e.g., into a needle array).

In some embodiments, emitters can be manufactured using methods thatallow for the monolithic building of emitters at the same time as thefabrication of the porous substrate from which the emitter is made. Forexample, emitters or emitter arrays can be formed by molding a poroussubstrate during its fabrication. Beneficially, for example, suchapproaches can facilitate alignment of the emitters and extractor gridearlier in the manufacturing process to mitigate error that can compoundthrough the manufacturing process.

FIG. 18 is a flowchart illustrating a method for manufacturingelectrospray emitters. In some embodiments, emitters (e.g., the emittersdescribed herein) can be manufactured using a sol-gel process. Forexample, emitter bodies can be made of a porous ceramic xerogelfabricated by a sol-gel process. As illustrated, at step 1805 a gelsolution is prepared. In some embodiments, a gel solution comprising asolvent, an acidic aluminum salt, a polymer, and/or a proton scavengercan be prepared. For example, to form an alumina xerogel, the gelsolution can comprise aluminum chloride hexahydrate, polyethylene oxide,water, ethanol, and propylene oxide. In some embodiments, the gelsolution can comprise 1 part by mass of polyethylene oxide, 50 parts bymass water, 54.4 parts by mass ethanol, 54.4 parts by mass propyleneoxide, and 54 parts by mass of aluminum chloride hexahydrate.

At step 1810, a mold for one or more emitter bodies is provided. Themold can be for forming emitter bodies that include a base and a tip.The mold can be made from polydimethylsiloxane (PDMS),polytetrafluoroethylene (PTFE), polymers, fluoropolymers, paraffin wax,silica, glass, aluminum, and/or stainless steel. In some embodiments,the mold can be polished with a smooth finish. In some embodiments,micro fabrication techniques can be used to create molds with micronsized features (e.g., for forming emitters). Silicon micro fabricationcan be used to form the positive for the mold. In some embodiments laserablation can be used to form small features (e.g., for forming emitters)in molds. At step 1815, the gel solution can be poured into the mold. Atstep 1820, the gel solution can be dried in the mold to form the emitterbodies from the porous ceramic xerogel. In some embodiments, the porousceramic xerogel made by the described process can include poresapproximately 3-5 μm in diameter.

FIG. 19 is a flowchart illustrating a method for manufacturingelectrospray emitters. In some embodiments, emitters (e.g., the emittersdescribed herein) can be manufactured using a freeze cast process. Forexample, emitter bodies can be made of a porous ceramic materialfabricated by a freeze cast process. As illustrated, at step 1905 aslurry is prepared. For example, a slurry can be prepared comprising aceramic component. The slurry can comprise at least silica, water, and aceramic component. In some embodiments, the slurry can comprise amixture of colloidal silica (e.g., LUDOX TM-50 sold by Sigma-AldrichCo.) and nanopowder alumina (e.g., calcined alumina A16 SG sold byAlmatis). In some embodiments, the ratio of alumina to silica can bevaried from 45-65% alumina and 55-35% silica by mass to vary the poresize. Deionized water, ethanol, isopropanol, glycerol, and/or othersolvents can be added to alter the porosity of the resulting porousceramic material. For example, the addition of glycerol can contributeto pore size uniformity. The slurry can be mixed to form a homogenous,white, and viscous liquid.

At step 1910, a mold for one or more emitter bodies is provided. Themold can be for forming emitter bodies that include a base and a tip.The mold can be made from polydimethylsiloxane (PDMS),polytetrafluoroethylene (PTFE), polymers, fluoropolymers, paraffin wax,silica, glass, aluminum, and/or stainless steel. In some embodiments,the mold can be polished with a smooth finish. In some embodiments microfabrication techniques can be used to create molds with micron sizedfeatures (e.g., for forming emitters). Silicon micro fabrication can beused to form the positive for the mold. In some embodiments, laserablation can be used to form small features (e.g., for forming emitters)in molds.

At step 1915, the slurry can be poured into the mold. At step 1920, theslurry can be frozen in the mold to form a frozen slurry (e.g., beingplaced into a bath of liquid nitrogen). At step 1925, the frozen slurrycan be freeze dried to form the emitter bodies made from the porousceramic material. For example, the frozen slurry can be placed in avacuum desiccator, where the ice particles in the frozen slurry areallowed to sublimate resulting in a porous ceramic material. In someembodiments, low temperature sintering can be used to increase thestrength of the porous ceramic material. In some embodiments, the porousceramic material made by the described process can include poresapproximately 3-5 μm in diameter. Beneficially, the described freezecasting process can be used for filling micron-sized capillaries withporous ceramic material. For example, the slurry can be allowed to soakup into the capillaries. The slurry, along with the substrate containingthe capillaries can be flash frozen and freeze dried as described above.

FIG. 20 is a flowchart illustrating a method for manufacturingelectrospray emitters. In some embodiments, emitters (e.g., the emittersdescribed herein) can be manufactured using a combination of sol-gel andfreeze cast processes. For example, xerogel particles can be used in afreeze cast process to form emitters. At step 2005, a gel solution isprepared. For example, a gel solution comprising a solvent, an acidicaluminum salt, a polymer, and a proton scavenger can be prepared. Atstep 2010, the gel solution can be dried to form porous ceramic xerogel.At step 2015, the porous ceramic xerogel can be ground to form groundporous ceramic xerogel (e.g., porous ceramic xerogel particles less thanapproximately 50 μm in diameter). The ground porous ceramic xerogel canthen be used in a freeze cast process. At step 2020, a slurry can beprepared comprising the ground porous ceramic xerogel. The slurry caninclude at least silica, water, and the ground porous ceramic xerogel.

At step 2025, a mold can be provided, the mold for forming emitterbodies that include a base and a tip. The mold can be made frompolydimethylsiloxane (PDMS), polytetrafluoroethylene (PTFE), polymers,fluoropolymers, paraffin wax, silica, glass, aluminum, and/or stainlesssteel. In some embodiments, the mold can be polished with a smoothfinish. In some embodiments micro fabrication techniques can be used tocreate molds with micron sized features (e.g., for forming emitters).Silicon micro fabrication can be used to form the positive for the mold.In some embodiments laser ablation can be used to form small features(e.g., for forming emitters) in molds. At step 2030, the slurry can bepoured into the mold. At step 2035, the slurry can be frozen in the moldto form a frozen slurry (e.g., being placed into a bath of liquidnitrogen). At step 2040, the frozen slurry can be freeze dried to formthe emitter bodies made from the porous ceramic material.

FIG. 21 is a schematic of mold 2105 for emitter bodies. Mold 2105includes emitter tip portion 2110 for forming a tip of an emitter andemitter base portion 2115 for forming a base of the emitter. Asdescribed above, material (e.g., a gel solution and/or a slurry) can bepoured into mold 2105 to form emitters (e.g., emitters having a bodywith a base and a tip). Mold 2105 can be formed as described above.

In some embodiments, a freeze cast process can be used to fabricateemitters having a pore size gradient that decreases from the base of theemitter to the tip emitter (e.g., as described above). For example, whenfreezing the slurry, a temperature gradient can be introduced to form agradient in the pore size of the resulting porous material. In someembodiments, the slurry can be placed on a thermally conductive base,which is then cooled. Ice particles nearer to the thermally conductivebase will freeze faster and result in smaller ice crystals than icecrystals further away from the thermally conductive base. During freezedrying, the smaller ice crystals can result in smaller pore size in theporous material than the larger ice crystals. In some embodiments, atemperature gradient that increases from the emitter tip portion of themold to the emitter base portion of the mold (e.g., as illustrated byarrow 2120 of FIG. 21) is applied to the slurry in the mold duringfreezing so that the resulting porous material has a pore size gradientthat decreases from the base of the emitter to the tip emitter. In someembodiments, a freeze cast process can be used to fabricate emittershaving a pore size gradient that decreases from the base of the emitterto the tip emitter by layering slurries of different compositions. Forexample, thin layers of slurry in the mold can be flash frozen (e.g., byusing liquid nitrogen). Additional layers of slurry (e.g., slurry with adifferent composition recipe that is conducive to larger (or smaller)pore sizes) can be freeze casted on the previous slurry layer. Thisprocess can be repeated until the desired porosity gradient is achieved.

Experimental Results

At the electrode-liquid interface of some ion sources, anelectrochemical double layer (DL) exists in a state of equilibrium. Ascurrent is supplied to or drawn from the electrode, its charge level ismodified, thus changing the potential difference across the DL. When theDL potential exceeds the electrochemical window limit of the ionicliquid, electronic charge can transfer across the interface (Faradiccurrent) with potentially damaging redox reactions. As described above,some embodiments of the invention can include a distal electrode (e.g.,distal electrodes 1322, 1424, and 1520) making electrical contactexclusively with the ionic liquid. Such a configuration can mitigateelectrochemical degradation of emitter tips. By moving the electricalcontact from, for example, the emitters to a distal electrode with arelatively large surface area, electrochemical reactions at the emittertips can be reduced, both in single emitters and dense arrays. The DLforming at the distal electrode can still saturate, but only after acomparatively long time. For example, for emitters spaced out in atwo-dimensional array by a distance p, saturation time will be on theorder of τ*˜τ (p/D)⁴ as the current distributes in the larger area ˜p².Similarly, the distal electrode can be as large as practical in singleemitters. In both cases, voltage alternation at modest frequencies canbe a technique for preventing electrochemical degradation.

As described above, in some embodiments, emitters can be fabricated froman insulating material (e.g., porous ceramic, glass or other oxidematerial). In these embodiments, there is no electrochemistry at theemitter surface as long as the electrochemical window is always lowerthan the equilibrium DL potential, which would not change as the localsurface potential would follow the potential of the liquid. In someembodiments, emitters can be fabricated from metal, but separated fromthe distal electrode by the ionic liquid itself. In these embodiments,the electric potential of the metallic emitter substrate would beconstant, while the potential difference across the DL would change as aresult of the ohmic drop along the effective liquid path l separatingthe distal electrode from the emission site near the micro-tips, givenby ΔV≈Il/KA. A is the effective cross sectional area through which thecurrent I is transported. As long as ΔV<<V_(w), electrochemicalreactions will not occur at significant rates along the liquid path incontact with the metallic emitter substrate.

An emitter was wetted with EMI-BF4 and passed through a stainless steelcylindrical reservoir serving as distal electrode (e.g., as illustratedin FIGS. 13A and 13B), such that its contact area with the liquid is 18mm². A square-wave voltage (±1285 V) was applied to the extractorelectrode at a frequency of 0.1 Hz. The emitter voltage with respect toground was monitored using a high impedance (>1 TΩ) electrometer. Theexposed emitter length, L, was varied from about 1 to 4 mm, in-situ. Theeffective liquid path was then l˜L. The current to the cylinder (equalto the ion beam current) was monitored by a second electrometer andvaried from 60-70 nA for short and long L, respectively. Experimentswere carried out in vacuum at pressures about 10⁻⁶ Torr.

FIG. 22 is a graph 2200 showing the relative variation of the emittervoltage immediately after polarity reversal. The relative variation ofthe emitter voltage immediately after polarity reversal (at t=0) isplotted for two exposed lengths. Eventually, the charging rates of theemitter in both cases become very similar and are consistent withuniform capacitive charging of the cylinder inner surface (˜0.04 V/s).The voltage offset between the two curves is due to a quick differenceof initial slopes. Its value (˜0.01 V) is also consistent with theanticipated ohmic drop (higher, for longer L) along the liquid layer onthe exposed surface.

While the invention has been particularly shown and described withreference to specific illustrative embodiments, it should be understoodthat various changes in form and detail may be made without departingfrom the spirit and scope of the invention.

1. (canceled)
 2. A method of forming an electrospray emitter comprising:pouring a gel solution into a mold shaped to form one or more emitterbodies with a base and a tip; and drying the gel solution in the mold toform the one or more emitter bodies at least partially from a porousmaterial.
 3. The method of claim 2, wherein the gel solution is a solgel solution.
 4. The method of claim 3, wherein the sol gel solutionincludes a solvent, an acidic aluminum salt, a polymer, and a protonscavenger.
 5. The method of claim 3, further comprising mixing aluminumchloride hexahydrate, polyethylene oxide, water, ethanol, and propyleneoxide to form the sol gel solution.
 6. The method of claim 2, whereinthe porous material is a porous xerogel.
 7. The method of claim 2,wherein the porous material is a ceramic.
 8. The method of claim 2,wherein the porous material is alumina.
 9. The method of claim 2,wherein a pore size of the one or more emitter bodies decreases from thebase to the tip of the one or more emitter bodies.
 10. The method ofclaim 2, wherein pores of the porous material have a diameter between orequal to 3 μm and 5 μm.
 11. The method of claim 2, wherein a radius ofcurvature of the tip of the one or more emitter bodies is between orequal to 1 μm and 20 μm.
 12. A method of forming an electrospray emittercomprising: pouring a slurry including ground porous material into amold shaped to form one or more emitter bodies with a base and a tip;freezing the slurry in the mold to form a frozen slurry; and drying thefrozen slurry to form the one or more porous emitter bodies.
 13. Themethod of claim 12, wherein drying the frozen slurry includesfreeze-drying the frozen slurry.
 14. The method of claim 12, wherein theground porous material is a ground porous ceramic.
 15. The method ofclaim 12, wherein the ground porous material is a ground porous xerogel.16. The method of claim 12, further comprising sintering the one or moreemitter bodies.
 17. The method of claim 12, wherein freezing the slurryfurther comprises freezing the slurry in the mold with a temperaturegradient such that a temperature of the slurry increases in a directionfrom the tip of the one or more emitter bodies toward the base of theone or more emitter bodies.
 18. The method of claim 12, wherein a poresize of the one or more emitter bodies decreases from the base to thetip of the one or more emitter bodies.
 19. The system of claim 12,wherein pores of the ground porous material have a diameter between orequal to 3 μm and 5 μm.
 20. The system of claim 12, wherein a radius ofcurvature of the tip of the one or more emitter bodies is between orequal to 1 μm and 20 μm.
 21. The method of claim 12, further comprisinggrinding a porous material to form the ground porous material.